Same RSV Drug, Very Different Results Across Europe

Regions that reached more infants with nirsevimab, a long-acting antibody injection given to babies to prevent respiratory syncytial virus, recorded the largest declines in emergency-department visits for bronchiolitis, according to an analysis of 107,088 cases across 12 European countries published on September 3 in PLOS Medicine. The authors write that because this is an ecological study, "causality cannot be inferred."

The EPISODES Study Group, led by Léa Lenglart of Robert Debré University Hospital in Paris, used routinely collected records from 27 pediatric emergency departments between January 2018 and March 2024, covering every bronchiolitis case in infants under 12 months. The team compared regions that had introduced nirsevimab against regions that had not, using controlled interrupted time-series models that account for seasonality and longer-term trends.

Across regions and age groups, the change during the October 2023 to March 2024 season ranged from a 0.6% increase to a 61.0% decline, the paper reports. Coverage, taken from official regional reports, tracked the size of that change, a correlation measured across regions rather than a trial. The authors add that unmeasured regional differences may have contributed to the variability.

Bronchiolitis trends in the control countries stayed stable over the same period, as did emergency visits for urinary tract infections, which the group used as a control outcome.

The authors conclude that coverage was "a key factor associated with bronchiolitis reduction," and that nirsevimab's benefit depends "not only on its intrinsic efficacy, but also on how effectively it is delivered." Nirsevimab was introduced across Europe in 2023, and the paper notes its effectiveness had already been shown in clinical trials and earlier real-world studies.